outwork vs ravelin

outwork

noun
  • A minor, subsidiary fortification built beyond the main limits of fortification. 

  • Agricultural work done outdoors in the fields. 

verb
  • To work out to a finish; to complete. 

  • To work faster or harder than. 

ravelin

noun
  • An outwork. A fortification outside a castle used to split an attacking force; composed of two faces, forming a salient angle whose gorge resembles a half-moon 

How often have the words outwork and ravelin occurred in a corpus of books? (source: Google Ngram Viewer )